This tool is Gentoo's Reverse Dependency rebuilder. It will scan your installed ebuilds to find packages that have become broken as a result of an upgrade of a package they depend on. It can emerge those packages for you but it can also happen that a given package does not work anymore with the currently installed dependencies, in which case you should upgrade the broken package to a more recent version. revdep-rebuild will pass flags to emerge which lets you use the --pretend flag to see what is going to be emerged again before you go any further.